This invention relates generally to antennas, and more particularly to multi-frequency antennas including a slot antenna.
Wireless communications technology today requires cellular radiotelephone products that have the capability of operating in multiple frequency bands. The normal operating frequency bands, in the United States for example, are analog, Code Division Multiple Access (CDMA) or Time Division Multiple Access (TDMA) at 800 MHz, Global Positioning System (GPS) at 1500 MHz, Personal Communication System (PCS) at 1900 MHz and Bluetooth(trademark) at 2400 MHz. Whereas in Europe, the normal operating frequency bands are Global System for Mobile Communications (GSM) at 900 MHz, GPS at 1500 MHz, Digital Communication System (DCS) at 1800 MHz and Bluetooth(trademark) at 2400 MHz. The capability to operate on these multiple frequency bands requires an antenna structure able to handle all these frequencies.
External antenna structures, such as retractable and fixed xe2x80x9cstubbyxe2x80x9d antennas have been used with multiple antenna elements to cover the frequency bands of interest. However, these antennas, by their very nature of extending outside of the radiotelephone and of having a fragile construction, are prone to damage. In particular, as the size of radiotelephones shrink, users are more likely to place the phone in pockets or purses where they are subject to jostling and flexing forces that can damage the antenna. Moreover, retractable antennas are less efficient in some frequency bands when retracted, and users are not likely to always extend the antenna in use since this requires extra effort. Further, marketing studies also reveal that users today prefer internal antennas to external antennas.
The trend is for radiotelephones to incorporate fixed antennas contained internally within the radiotelephone. However, this typically increases the size of the radio telephone to accommodate the antenna structure, and it is difficult to maintain antenna efficiency, since the antenna element are now placed in proximity to other conductive components in the radiotelephone. Moreover, the antenna is more susceptible to interference from these same conductive components, further impairing efficiency, particularly in the low frequency bands.
Slot and microstrip transmission line antennas can be used in high frequency applications and have a very low profile. However, due to size constraints, these antennas can only operate in one single frequency band. Slot antennas can be implemented with cutout in a metal surface. Prior art resonant slot antenna geometries include a half wavelength (xcex/2) full slot antenna where both ends of the slot are closed, and the length of the slot is a half wavelength (about 80 mm at 1800/1900 MHz, which is quite long and not practical for cellular phone). Another type of slot antenna is a one-quarter wavelength (xcex/4) open-end slot antenna 10 as shown in prior art FIG. 1. For a xcex/4 slot antenna 10, the length 12 of the slot 14 is a quarter wavelength with one end of the slot 14 closed while the other end is open. The slot 14 is excited differentially by energy coupled from an excitation port providing a positive charge 13 and a negative charge 15 near the closed end of the slot 14 and perpendicular to the slot as shown. The excitation port is typically provided by a microstrip line embedded under the slot. A conductive ground plane 16 surrounds the slot 14. More than one slot antenna can be used in a radiotelephone to obtain radiation in multiple frequency bands. However, separate antennas require separate excitation ports and individual electronic tuning mechanisms, which increases size and cost.
Therefore, there is a need for a small size and low cost internal antenna apparatus with and multi-band frequency radiation capability. Another desired advantage would be to provide performance comparable to external multi-band antennas. It would also be of benefit to provide this antenna apparatus driven by a single excitation port.